MIMO technology offers the possibility of significantly increasing the transmission capacity of a radio channel. It is a technique which is extensively used in radio access systems such as wireless local area networks, WLANs, and long-term evolution, LTE, mobile networks.
MIMO technology can also be applied to increase transmission capacity in high frequency point-to-point microwave radio links. This type of MIMO system as a rule uses highly directive antennas and operates in line-of-sight, LOS, conditions. A MIMO system operating under these conditions is often referred to as a LOS-MIMO communication system.
A difference between LOS-MIMO systems and traditional MIMO systems is that a LOS-MIMO system, having highly directive antennas and operating in LOS conditions, as a rule cannot make use of multipath propagation in order to perform the MIMO signal processing required to recover transmitted data. Instead, a LOS-MIMO communication system relies on a diversity of phase shifts along the different propagation paths between transmitter and receiver antennas, and not on multipath propagation. These phase shifts must satisfy certain conditions in order for a LOS-MIMO receiver to be able to recover transmitted data, i.e., the phase shifts must together provide a MIMO channel matrix which has full rank.
Thus, in order for a LOS-MIMO communication system to function as intended, inter-antenna distances must be carefully set as a function of transmission frequency in order to provide suitable phase shifts along the different propagation paths between transmit and receive antennas, which phase shifts allow a LOS-MIMO receiver to recover transmitted data.
Consequently, a correct antenna deployment is necessary in order for a LOS-MIMO communication system to be able to provide said increase in transmission capacity.
A problem then, is how to find such a suitable LOS-MIMO antenna deployment for a given environment and transmission frequency band.
One possible approach to evaluating a LOS-MIMO antenna deployment in order to see if it is suitable for MIMO communication is to actually deploy the full LOS-MIMO communication system including antennas, modems and power supply, and then measure the phase differences between the different propagation paths, which phase differences can typically be calculated by a LOS-MIMO system receiver or digital signal processor, DSP.
A drawback with this approach is that the MIMO communication system has to be fully installed and operational, meaning that any subsequent modification of antenna positions is likely to be difficult and expensive to accommodate. Also, a sufficiently good antenna deployment must be achieved from the start in order for the MIMO system receiver to be able to acquire the transmitted signal and measure said phase differences.
Another approach is to measure the exact locations in three dimensions of a planned set of antenna positions using, e.g., a Global Positioning System (GPS) tool and then calculate phase differences between said propagation paths from the inter-antenna distances and the transmission frequency. However, in reality, it can be difficult to obtain sufficient accuracy in such GPS measurements, e.g., due to that antenna locations are at positions with an obscured view of the sky. Also, this approach does not account for phase shifts which are due to effects other than phase shift from propagation distance, e.g., phase shift effects due to multipath and other environmental effects on received phase.
Consequently, there is a need for a tool which allows quick and cost-effective, and yet precise, evaluation of the suitability of a given set of antenna positions, i.e., a LOS-MIMO antenna deployment, for use in a LOS-MIMO communication system.